Polymorphism and phase transitions in NaUO from density functional perturbation theory.

Polymorphism and phase transitions in sodium diuranate, NaUO, are investigated with density functional perturbation theory (DFPT). Thermal properties of crystalline α-, β- and γ-NaUO polymorphs are predicted from DFPT phonon calculations, , the first time for the high-temperature γ-NaUO phase (3̄ symmetry). The standard molar isochoric heat capacities predicted within the quasi-harmonic approximation are for 2/ α-NaUO and 2/ β-NaUO, respectively.

Control of Structural Hydrophobicity and Cation Solvation on Interlayer Water Transport during Clay Dehydration.

Swelling clay hydration/dehydration is important to many environmental and industrial processes. Experimental studies usually probe equilibrium hydration states in an averaged manner and thus cannot capture the fast water transport and structural change in interlayers during hydration/dehydration. Using molecular simulations and thermogravimetric analyses, we observe a two-stage dehydration process.

Gibbs Energy Minimization Model for Solvent Extraction with Application to Rare-Earths Recovery.

The emergence of technologies in which rare-earth elements provide critical functionality has increased the demand for these materials, with important implications for supply security. Recycling provides an option for mitigating supply risk and for creating economic value from the resale of recovered materials. While solvent extraction is a proven technology for rare-earth recovery and separation, its application often requires extensive trial-and-error experimentation to estimate parameter values and determine experimental design configurations.

Nonlinear dynamics and instability of aqueous dissolution of silicate glasses and minerals.

Aqueous dissolution of silicate glasses and minerals plays a critical role in global biogeochemical cycles and climate evolution. The reactivity of these materials is also important to numerous engineering applications including nuclear waste disposal. The dissolution process has long been considered to be controlled by a leached surface layer in which cations in the silicate framework are gradually leached out and replaced by protons from the solution.

On the role of strong electron correlations in the surface properties and chemistry of uranium dioxide.

We report density functional calculations of the surface properties and chemistry of UO(2)(111) performed within the generalized gradient approximation corrected with an effective Hubbard parameter (GGA + U within Dudarev's formalism) to account for the strong on-site Coulomb repulsion between U 5f electrons. The variation of the properties of periodic slab models, with collinear ferromagnetic and antiferromagnetic arrangements of the uranium magnetic moments, was investigated while ramping up the effective Hubbard parameter from U(eff) = 0 eV, corresponding to standard density functional theory, up to U(eff) = 4 eV, the value that correctly reproduces the antiferromagnetic ground state of bulk UO(2). The chemical interactions of molecular water, dissociated water, dissociated oxygen and co-adsorbed molecular water and monatomic oxygen with the UO(2)(111) surface were also studied as functions of the U(eff) parameter.

Structures of uranyl peroxide hydrates: a first-principles study of studtite and metastudtite.

The structures of the only known minerals containing peroxide, namely studtite [(UO(2))O(2)(H(2)O)(4)] and metastudtite [(UO(2))O(2)(H(2)O)(2)], have been investigated using density functional theory. The structure of metastudtite crystallizing in the orthorhombic space group Pnma (Z = 4) is reported for the first time at the atomic level and the computed lattice parameters, a = 8.45, b = 8.